This invention relates to a resin composition comprising a thermoplastic polyester resin, a polyester elastomer, and vinilidene chloride resin (hereinafter called as "PVDC"), and a biaxially stretched heat shrinkable film including at least one layer thereof.
In general, shrink packaging is the most convenient form for uneven and irregular food products on their shapes, like raw meat or processed meat. As these food packaging needs a long preservation period, it is not only requested the superior oxygen gas barrier property [below 200 cc/m.sup.2 .multidot.day.multidot.atm. at 30.degree. C., under 100% RH], but also the superior cold resistance.
PVDC film is widely used with its superior properties on oxygen gas barrier property, oil resistance, and clipping property besides the shrinkability. However, ordinary PVDC film shows sometimes insufficient strength, especially, the strength under low temperature (cold resistance) under severe packaging conditions while packaging heavy materials, so it is desired to get the packaging materials without these defects.
Resin composition containing PVDC and polyester copolymer resin derived from terephthalic acid and ethylene glycol and cyclohexane dimethanol are disclosed in U.S. Pat. No. 4,725,651 and it is shown that the moldings gained, have superior oxygen gas barrier property and mechanical strength.
However, the film made by the blended resin composition comprising PVDC and polyester, can not satisfy yet on the point of its cold resistance. Some attempts, therefore, are carried out to get the heat shrinkable films having good oxygen gas barrier property and cold resistance with laminating PVDC barrier layer and other resin layers, for example, three-layers film are provided to improve the cold resistance with co-extruding to prepare three-layer product having intermediate PVDC layer containing extremely small amount of the additives like plasticizers and stabilizers or without any these additives, and two outer layers of ethylene-vinylacetate copolymer (EVA) having adhesive property onto PVDC and the excellent cold resistance, these outer layers being provided on both sides of the PVDC layer (Canadian patent No. 982923).
On the other hand, heat shrinkable laminated film which does include one PVDC layer and has not only oxygen gas barrier property and excellent cold resistance, but also high performance with melt hole resistance, heat-resistant sealing property (the heat resistance of a seal portion), and transparency after shrinking, is also required, especially for the packaging on fatty foods like processed meat and cheese. The packaging and sterilization of fatty foods often involves problems in that a film softened by oil and heat may be stretched to become thin and thus break (melt hole) and in that a sealed part or parts in the vicinity thereof may be broken owing to heat shrinkage stress during sterilization. Concerned industry, therefore desires eagerly the heat shrinkable film which has not only oxygen gas barrier property, but also melt hole resistance, heat resistance sealing property, sufficient cold resistance without occurring pinhole during low temperature distribution, and superior transparency after shrinking. As these heat shrinkable laminated films, laminates crosslinked by electron beam are used, for example, flexible laminate suitable for the heat shrinkable packaging contains (1) the first layer containing organic polymer, (2) the oxygen gas barrier layer based upon PVDC, having oxygen gas transmission rate of lower than 70 cc/m.sup.2 .multidot.day.multidot.atm., in oxygen gas transmission rate of the laminated film (following ASTM Standard D1434, at 22.8 .degree. C. and 0% in RH), (3) the layer containing organic polymer having endurance for abusing. And this three layer laminated film described above are known in U.S. Pat. No. 3,741,253 with following embodiment, in which a laminate is characterized by the first layer containing an oriented copolymer of ethylene and vinylacetate having 5.about.20% units in weight, derived from vinylacetate, and crosslinked said copolymer by the irradiation, the oxygen gas barrier layer (2) containing 70.about.85% units in weight, derived from vinylidene chloride and 30.about.15% units in weight, derived from vinyl chloride, and the layer (3) consisting of (i) copolymer of ethylene and vinylacetate including 5.about.20% units in weight derived from vinylacetate, (ii) polymer blend of isotactic polypropylene, atactic polypropylene, and polybutene-1. Also, there are some technology in this field, in which U.S. Pat. No. 4,699,846 dlscloses the improvement on peeling off resistance among layers on laminated film having (1) base film layer containing .alpha.-mono olefin polymer irradiated and crosslinked, (2) film layer containing polymer being crosslinked by the irradiation, and then whole layer (1) and (2) being irradiated to produce further crosslinking and stretched the laminated film to attain the object described above, Japanese Patent Application Laid Open No. 3948/1987 discloses the heat shrinkable biaxially stretched multilayer film being suitable for packaging on slices of primal and subprimal meat and processed meat with the multilayer film containing vinylidene chloride-methylmethacrylate copolymer as oxygen gas barrier layer and irradiated with 1.about.5 Mega rad dose level, and Japanese Patent Application Laid-Open No. 23752/1987 discloses molecular oriented multilayer polymer film having the first layer and the second layer consisting of mainly ethylene vinylacetate copolymer and the layer of PVDC composition disposed between the first and the second layers and each composition being irradiated by electron beam over 1.5 Mega rad to attain crosslinking bonding. However, when the resin composition in U.S. Pat. No. 4,725,651 is used as a single layer of heat shrinkable film having oxygen gas barrier property and cold resistance, it has some defects which are inferior pin-hole resistance at low temperature adopted in food distribution step, because of high glass transition temperature on polyester copolymer and have low productivity derived from violent degradation of the resin at extrusion molding. As the heat shrinkable laminated film which request high performance, for example, EVA/PVDC/EVA composition shows superior laminated film having heat sealable, cold resistance, and good oxygen gas barrier property and compensates the defects on PVDC single layer product, but it shows poor melt hole resistance and heat sealing resistance. Further, the stretching is done at the temperature not over 40.degree. lower than its crystal melting point of said polyolefin, to give shrinkability to a pair of polyolefin layer existing both side onto PVDC layer, so PVDC layer is not given enough stretching and orienting effect. Therefore, the heat shrinkability of PVDC layer is deficient, and when the laminate is heat shrinked, PVDC layer would be left behind in the shrinking behavior, and become to fine wrinkles after shrinking. In the result, the laminate film has a tendency to cause extremely poor transparency after shrinking. It is strongly desired, therefore, in the food packaging field, to provide the heat shrinkable laminated film with still excellent transparency after shrinking, having not only oxygen gas barrier property and excellent cold resistance, but also excellent melt hole resistance and heat resistance sealing property. Inventors have found out that the heat shrinkable film satisfied all requests described above is provided with using the resin composition comprising 26.about.70 weight % of thermoplastic polyester resin, 10.about.30 weight % of polyester elastomer, and 20.about.44 weight % of PVDC as the composition of the film having oxygen gas barrier property and cold resistance and attained this invention based upon this knowledge.